The tired old trope "erotic thriller" does no justice to how confrontationally and explicitly sexual this movie is — nor how thrilling, nor how menacing and complex. Alain Guiraudie's L'Inconnu du lac, or Stranger by the Lake, is a psychological suspense drama set at a French lakeside cruising spot for gay men. Throughout the summer, guys drive up; they park in some patchy clearing, then walk down to the pebbly lakeside in singles and couples. They sunbathe, but there are no books or magazines or Kindles: the name of the game is returning each other's glances. They go swimming, mostly naked, and then stroll back into the surrounding woodland, for casual sex, bareback or with condoms, while other guys look on, masturbating.

Guiraudie creates an atmosphere of absolutely frank homoeroticism, utterly without inhibition or self-consciousness or taboo: I was reminded of Alan Hollinghurst's The Swimming Pool Library or Thom Gunn's poem The Discovery of the Pacific. But when a single, terrible event takes place, the mood swings to that of classic Hollywood suspense, like John M Stahl's Leave Her to Heaven (1945) or George Stevens's A Place in the Sun (1951), movies in which a beautiful lake becomes the epicentre of danger.

Christophe Paou plays Michel, a handsome, well-built guy who comes to the lake in the company of someone who appears to be his boyfriend, although the relationship is tenuous. He is instantly enamoured of Michel, played by Pierre Deladonchamps, a slighter, more reserved and thoughtful man. As it happens, Michel has aready struck up a tender, platonic friendship with Henri (Patrick d'Assumçao), a fat, lonely and unhappy guy who sits all by himself, way away from all the others. He says that he has just split up from his girlfriend and seems to be believe that the number of men who are entirely gay, without any bi feelings, is very small.

This nexus of relationships, a sex-triangle, is placed under intense scrutiny when the police are called in to investigate a certain terrible event. A certain inspector finds himself perplexed and frustrated by the cruising credo: guys who are so into voyeurism should make good witnesses, but those who don't ask each other's names or phone numbers, guys who cultivate a willed forgetfulness about yesterday's experience so as to prepare the way for the next contact — they are creating a cloud of unknowing, highly injurious to a police investigation. But the film does not in fact suggest the crime is a part of any gay "culture" here — it might be instructive to show the film in a double-bill with William Friedkin's Cruising (1980). It is more a matter of a single individual's feelings, and this individual's complex motivation, made up of fear, love, desire and disbelief.

Guiraudie's sheer frankness about sex is refreshing when so many supposedly grownup films are content to hint coyly and timidly. The cruising scene takes it far away from any Joe Eszterhas-type sleaziness or supposed erotic danger. What is notable is how he manages to make the cruising itself in this movie look utterly free from coercion or menace. It is an almost pastoral scene, which makes the single violent act, and the reaction to it, so disturbing. I wondered if Guiraudie lost control of his movie at the end, just a little, but this utterly gripping and absorbing movie is one of the best at this year's London film festival.

Stephanie Theobald: Cinema and LGBT love: so often a mismatch. Two French films at this year's London film festival – Blue is the Warmest Colour and Stranger by the Lake – get me hot under the collar in different ways